Assertion failure int event-msw.c

On repeated exposure events in the Cygwin build I get an assertion failure in mswindows_dequeue_dispatch_event. I've never looked at this code before so there is little chance I could resolve the issue by the deadline for the next beta. If no one else can get a quick handle on the problem I will look more closely fore the following beta. I found it easy to reproduce the problem in gnus by simply getting into a newsgroup and repeatedly covering part of the Xemacs frame and then minimizing the covering window.